Microbial Pre-treatment of Double Refractory Gold Ores
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The use of microorganisms notably bacteria in mineral processing industry is presently one of the leading emerging pre-treatment techniques being employed for the processing of double refractory gold ores and concentrates. Currently numerous studies are in progress to further improve upon the efficiency of the bacterial process and to investigate the potential of other microorganisms. In this study, microbial pre-treatment of double refractory gold ore (sample A) and concentrate (sample B) was investigated using a white-rot fungus, Tramestes versicolor (ATTC 20869). Pulp density, temperature, pH and retention times were the process variables considered. Preliminary studies investigated the amenability of selected pure sulfide sulfur minerals, various types of coal and ore sample A to fungal degradation. Various pre-treatment scenarios were also studied to optimize gold extraction. These were single stage, a two-step and two stage processes involving the well known chemolithotrophic bacteria Acidithiobacillus thiooxidans (ATTC 15494), Acidithiobacillus ferrooxidans (ATTC 19859) and Leptospirillum ferrooxidans (ATTC 53992), the bacterium, Streptomyces setonii (ATTC 39116) and the white-rot fungus, Tramestes versicolor. Preliminary results for sample A indicated that T. versicolor did not degrade sulfides significantly at its optimum growth conditions (pH range of 4.5 – 5.0) and carbonaceous matter was not degraded but rather passivated as preg-robbing decreased significantly. Lignite was inert to passivation by T. versicolor unlike bituminous coal and to lower extent anthracite. Stimulated alkaline conditions (pH range of 9.5 – 10.5) recorded the overall best sulfur oxidation. Results of both the single stage and two-step processes confirmed that carbonaceous matter was passivated by T. versicolor; as preg-robbing decreased significantly from 18.1% to ≤ 1.0%. Gold extraction by cyanidation of the pre-treated sample A was 82.5% for the two-step as against 80.5 % for the single stage and 15.0% for the untreated sample. Application of the two-step and single stage process conditions to sample B resulted in 93.3% and 89.9% gold extraction respectively as against untreated concentrate of 30.5%. For the various two stage pre-treatment processes investigated, the abiotic - S. setonii process recorded the best gold extraction of 81.5% for sample A. S. Setonii degraded carbonaceous matter unlike T. versicolor which passivated it. However, it takes only 3 - 7 days for T. versicolor to effect passivation and eliminate preg-robbing while 14 - 56 days is required for S. setonii to degrade carbonaceous matter significantly. The result of the novel microbial pre-treatment process indicated that sulfide sulfur was degraded under alkaline conditions and carbonaceous matter passivated by T. versicolor at its optimum growth conditions. This led to a significant improvement in gold extraction from the double refractory gold ore and concentrate investigated.